It is generally the objective of seismic exploration to generate seismic energy, make measurements of and record the amplitude of any reflected and refracted energy at selected locations and for selected times, and then by selectively processing the recorded seismic data, to deduce the geometry of the subsurface geologic boundaries as well as some properties of the materials of the earth through which the seismic energy has propagated and from which it has been reflected.
Conventional land seismic acquisition techniques involve the use of an appropriate source (dynamite, vibrator(s), airguns(s), etc.) to generate seismic energy and a set of detectors, spread out on the surface of the earth, to detect any seismic signals due to seismic energy interacting with subsurface geologic boundaries. These detected signals are recorded as a function of time and subsequent processing of these signals, i.e., seismic "traces" or seismic data, is designed to reconstruct an appropriate image of the geologic boundaries of the subsurface and to obtain information about the subsurface materials.
Conventionally, the detector employed to detect seismic signals on land is a geophone. A geophone is an electro-mechanical device that is coupled to the ground via an extension or "spike" that is physically inserted into the ground. This allows the geophone case to vibrate because of any earth motions, including seismic signals. Internal to the geophone case and vibrationally isolated from the case (typically by springs) is an "inertial" mass that does not vibrate with the earth. Thus, there is a small relative motion between the geophone case and its inertial mass due to any detected ground motions. This relative motion is converted to an electrical signal by having a coil of wire that moves through an electromagnetic field of a permanent magnet; the magnet may be the inertial mass with the coil attached to the geophone case, or vice versa. This electrical signal is the seismic signal that is recorded (alone, or more preferably, in combination with other electrical signals) and later processed.
Seismic field layouts vary with the exploration objective sought to be detected. However, there is usually a need to simultaneously record seismic motion at many ground positions spaced over a wide area. A seismic line usually consists of multiple detector stations with each detector station having several geophones whose output signals are combined to form a single signal (array signal) for the detector station. The geophones are arranged in an array to reject unwanted waves while enhancing the reception of desired seismic waves on the electrical signals.
The signals from these arrays (detector stations) are collected and recorded for each seismic shot (seismic energy source). Conventional land seismic data acquisition is slow, expensive and labor intensive. In particular, the activity of laying out a cable which interconnects the detector stations, so that the array electrical signals may be recorded, is time consuming and requires a substantial work force. It would be desirable to have a fast way to move the entire seismic line or portions thereof and thereby minimize survey time.
A radar system which senses seismic ground motion (vibrations) remotely is one solution for reducing the cost and time for acquiring seismic data. Two such systems are described in U.S. Pat. No. 5,070,483, Remote Seismic Sensing, and U.S. Pat. No. 5,109,362, mote Seismic Sensing. Both patents use laser Doppler heterodyne interferometry techniques to detect the movements of the earth.
Another laser Doppler interferometry system is described in U.S. Pat. No. 4,284,350, Laser Geophone, which uses a homodyne system with a detector having side-by-side corner-cube retroreflectors for detecting vertical ground motions at a remote location. The use of a single or a side by side corner-cube retroreflector arrangement is also suggested for some detectors in embodiments of the systems described in the before mentioned U.S. Pat. Nos. 5,070,483 and 5,109,362.
Simplified illustrations of the prior art Laser Geophone (detector) of U.S. Pat. No. 4,284,350 are provided in FIGS. 1A and 1B. FIG. 1A is a front view showing two side-by-side corner-cube retroreflectors 2, 4. The prior art detector modulates two approximately horizontal laser beams (a first and second sensing beam) which illuminate the detector. The sensing beams are reflected at the detector downward by a mirror 6 onto the side-by-side (first and second) corner-cube retroreflectors 2, 4. The first sensing beam is reflected from the mirror 6 to the first retroreflector 2; the second sensing beam is reflected from the mirror 6 to the second retroreflector 4. The mirror 6 and the first retroreflector 2 are coupled to the motions of the ground, i.e., they are seismically coupled to the motions of the earth; the second retroreflector 4 is inertially isolated from the motions of the ground in the vertical direction.
Each sensing beam is frequency modulated by the detector. When a sensing beam is incident normally on an optical component which reflects (or deflects) the path of the incident sensing beam (such as the mirror 6 or retroreflectors 2, 4), the frequency of the beam may be frequency modulated (Doppler shifted). The amount of Doppler shift at each reflection point is proportional to the relative velocity of the reflection point on the Doppler shifting optical component with respect to the incident path of the beam which strikes that point. As the beam is reflected through the detector, the Doppler shifts are cumulative; thus, the Doppler shifts add or subtract from the previous Doppler shifts on the beam. Upon exiting the detector, if the points of reflection on the combination of Doppler shifting optical components have undergone a net relative movement along the incident paths of the sensing beam, the cumulative movement of the reflection points would represent a net velocity with respect to the sensing beam. And, the sensing beam will be Doppler shifted (frequency modulated) by an amount which represents the net velocity of these reflection points relative to the incident paths of the sensing beam.
In this specification, an orthogonal coordinate system is used. The terms "vertical" and "horizontal" are used with respect to the motions of the earth at the detector. Vertical defines one ordinate for motions back and forth or up and down along the vertical ordinate. The horizontal is further defined with respect to the path of the transmitted sensing beam to identify a horizontal-inline ordinate and a horizontal-crossline ordinate. A horizontal line, formed by intersecting a vertical plane which extends in the inline direction of a transmitted beam with a horizontal plane, defines the horizontal-inline ordinate and the horizontal-inline direction. A horizontal line perpendicular to the horizontal-inline ordinate defines the horizontal-crossline ordinate and horizontal-crossline direction.
Returning to the prior art detector, FIG. 1B shows a side view of the prior art detector of FIG. 1A. Mirror 6 is shown to be at approximately a 45 degree angle with respect to vertical direction and parallel to the horizontal-crossline direction. Since the mirror 6 is coupled to the motions of the ground, the movement of the mirror 6 with respect to the path of both sensing beams imparts a Doppler shift on the sensing beams each time the sensing beams reflect from mirror 6; these Doppler shifts will represent motion in two directions, the relative vertical movement of the mirror and the horizontal-inline movement of the mirror with respect to the incident paths of the beams. Here, no Doppler shift is contributed to the beams for horizontal-cross line movement by the mirror 6 because the mirror is parallel to the horizontal-cross line direction. In the horizontal-crossline direction, the mirror has no relative movement with respect to the incident paths of the beam; thus, these motions do not Doppler shift the beams.
Returning to FIG. 1A, each sensing beam upon initial reflection by the mirror 6 enters the aperture of their respective corner-cube retroreflector 2, 4. A corner-cube (trihedral) retroreflector has the property that any ray entering the effective aperture will be reflected and emerge from the entrance/exit face parallel to itself, but with an opposite direction of propagation. An incident beam, hitting the effective aperture, is reflected exactly back on itself. These properties are, within acceptable angle limits, independent of the orientation of the corner-cube retroreflector. Because the incident beam is reflected within the retroreflector such that the reflected output beam is reflected exactly back on itself, the individual rays of the reflected beam are parallel to their initial position in the incident beam which strikes the retroreflector. This characteristic of the retroreflector is due to the structure of a corner-cube retroreflector; it has three mutually perpendicular reflectors and each ray entering the retroreflector will reflect from each reflector before exiting.
Each of the reflectors move in the same direction, but because they face each other, each reflector has a different relative motion with respect to an incident beam striking it from the previous reflector. Since each ray reflects from each mutually perpendicular reflector, the net relative movement for lateral motions on the corner-cube with respect to that ray is zero; thus, the Doppler shifts, placed on each ray for the lateral motions on each reflector as it reflects through the corner-cube, cancel. In this case, the lateral motions on retroreflectors 2, 4 are motions in the horizontal-inline and horizontal-crossline directions.
However, for motions back and forth along the path of the beam from the mirror 6 (here, the vertical direction), the net relative motion is not zero and the beam will be Doppler shifted to reflect this motion.
After each sensing beam is reflected by the retroreflectors 2, 4, they are again reflected (and Doppler shifted), as discussed previously, by the mirror 6. Note that the Doppler shifts imposed on each beam by the mirror are additive. This can be seen by assuming that the mirror is moving upward, the upward motion of the mirror at the 45 degree angle lengthens the path the initial incident beam takes before reflecting from the mirror. The same phenomenon occurs for the incident beam reflected from a retroreflector (the upward motion of the mirror 6 also tends to lengthen the path of the incident beam from the retroreflector); thus, the mirror has a net relative motion in the vertical direction with respect to each beam of FIG. 1A. A similar analysis could be made for mirror motions in the horizontal-inline direction to determine that the mirror 6 has a net relative motion in the horizontal-inline direction.
However, the relative vertical motions of the mirror 6 and retroreflector 2 are in opposite directions with respect to the path of the first sensing beam through the detector of FIG. 1A. This can be seen by noting that when mirror 6 is moving vertically upward, retroreflector 2 also moves in an upward direction. But, the upward motion of retroreflector 2 shortens the incident path of the beam from the mirror; whereas, the upward motion of the mirror lengthens the incident paths of the beams that strike the mirror; thus, the (vertical motion) Doppler shifts imposed on the beam by the retroreflector are opposite to the cumulative Doppler shifts imposed on the beam by the mirror for vertical motions. Since the cumulative vertical Doppler shifts from the mirror 6 are approximately equal to the Doppler shifts from the retroreflector 2, these vertical Doppler shifts cancel. Thus, the frequency modulated first sensing beam of this example contains Doppler shifted frequency components which represent only horizontal-inline ground motions at the Laser Geophone.
The frequency modulated second sensing beam also contains Doppler shifted frequency components because of the motions of mirror 6. However, these frequency components represent vertical ground motions as well as horizontal-inline ground motions. This is because, the inertially isolated retroreflector 4 only acted to reflect the beam; it had no motion in the vertical direction; thus, it did not add to or cancel any Doppler shifted frequency components on the second sensing beam.
In theory, this type of side-by-side retroreflector detector provides Doppler shifted frequency components on the two modulated sensing beams which are approximately identical (common mode signals). Here, the common mode signals represent the horizontal-inline motions on the detector. In addition, this prior art configuration also provides frequency components which are not common to the two beams. In this particular case (the Laser Geophone system), when the modulated beams are combined by optical homodyning, the common mode signals cancel each other and a difference signal (the frequency components which are not common to both beams) remains. One problem with a homodyning system is that it is not possible to determine "up" Doppler and "down" Doppler motion from the obtained difference signal. This problem is resolved by using a heterodyning system instead. However, in spite of this problem, the difference signal provided by this prior art homodyning system does represent the vertical motions of the earth at the remote location.
A side-by-side corner-cube retroreflector configuration, as mentioned, is also suggested for some embodiments of the prior art heterodyning systems of previously mentioned U.S. Pat. Nos. 5,070,483 and 5,109,362. However, the configurations taught in these patents do not deflect or reflect a sensing beam onto retroreflectors.
These systems, however, require at least one sensing beam and return beam for each detector (geophone equivalent). An array may have as many as 32 geophones for each detector station and a seismic line may have a hundred or more detector stations. If 3-D detection is desired, there could be many additional seismic lines. The number of sensing and return beams necessary to acquire this data by a remote sensing system could be substantially reduced if only one sensing beam and return beam are used for each detector station.
Also, the positioning of a remote detector is determined by maintaining a line of sight between each remote detector and the sensing beam transmitter and receiver. It may not be possible due to local obstacles to arrange remote detectors into an optimum array configuration for each detector station.
In addition, small seismic motions may not be detectable by these prior art systems. A large seismic motion (motion of the earth) induced in exploration surveys may be on the order of 1.times.10.sup.-2 m/sec so the maximum Doppler shift at the higher conventional laser frequencies is around 30 kHz (Fdop=2 Vseis/Wavelength, where Fdop is the Doppler shift, Vseis is the relative velocity (cumulative velocity) at the remote location and Wavelength is the wavelength of the carrier frequency of the transmitted laser beam). A small seismic motion which is capable of producing a Doppler on a sensing beams may be on the order of 2.0.times.10.sup.-6 m/sec. This small seismic motion on the above system would produce a Doppler shift of about 6.0 Hz. A difference signal having a Doppler shift this small may not be detectable due to equipment limitations, e.g., frequency drift and equipment noise, or to turbulence induced frequency fluctuations.
Turbulent noise may affect the side-by-side corner-cube retroreflector configurations of the before mentioned designs, or any other type of side-by-side retroreflector design. This is because this type of remote detector necessarily requires that two sensing beams (taking into consideration line of sight between the locations) travel different paths through the air to and from the retroreflectors. Separate air paths may have different effects upon the propagating laser sensing beams. Solar radiation heats the ground surface, causing convective air currents which break into turbulent flow. These randomly sized (roughly 1 millimeter to 1 meter) air packets have anomalous temperatures and refractive indices. The optical phase of each laser beam shifts as it passes through a region of anomalous refractive index. These air packets blow across the raypath and cause time-varying, random frequency modulation of the laser beam. Thus, two laser beams traveling through different air spaces will experience different fluctuations in each carrier frequency of the laser beams.
Since, the two laser beams are spatially separated, they will not be affected equally by the atmosphere and the atmospheric effects on the beams are not totally common mode signals. The atmospheric effects do not completely cancel when the two laser beams are combined by optical homodyning or heterodyning or by electronic homodyning when the beams are converted to electrical signals. Thus, the difference signal will not only contain the desired Doppler signal but also an additional component which will be referred to herein as turbulent noise. Turbulent noise is especially prevalent on sunny, windy days. Consequently, if side by side retroreflectors are used as parts of the detector in a remote seismic sensing system, the presence of turbulent noise on the difference signal could prevent an accurate determination of the desired (or selected) ground motions (the desired Doppler signal).
If the amount of Doppler shift representing the desired difference signal could be increased, seismic signals having smaller amplitudes could be detected. The increased Doppler shift would ensure that the difference signal is greater than the inherent equipment limitations of the remote detecting system. In addition, the increased Doppler shift of the desired difference signal would be much larger than the Doppler shift contributed by the turbulent noise; thus, the relative amount of turbulent noise present on the difference signal could be substantially reduced.